Photochemical Transformations and Laser Flash Photolysis Studies of Dibenzobarrelenes Containing 1,2-Dibenzoylalkene Moieties

Article abstract of DOI:10.1021/jo00199a035

Phototransformations of the dibenzobarrelenes 10, 11, and 23, prepared by the reaction of the appropriate anthracenes with dibenzoylacetylenes, are reported.Irradiation of 10 and 11 in solvents such as benzene, methanol, and acetone gave the corresponding dibenzosemibullvalenes, 17 and 18, respectively, in good yields.These dibenzosemibullvalenes on catalytic hydrogenation using 5 percent Pd on charcoal gave the corresponding cyclopropane ring-opened products, 21 and 22, respectively.Irradiation of 23 in benzene gave a mixture of 2,3-dibenzoyl-2,3-dihydro-1,4-dimethyl-2,3- benzonapththalene (27, 12 percent), 1,4-dibenzoyl-5,8-dimethyl-2,3:6,7-dibenzocyclooctatetraene (24, 20 percent), the carbinol 29 (45 percent), and benzoic acid (7percent).Irradiation of 23 in methanol gave a mixture of 27 (25 percent), 24 (27percent), and benzoic acid (8percent).Laser flash photolysis (337.1 nm) of 10, 11, and 23 led to transient species, characterized by strong absorptions in the 300-650-nm region.These transients are due to the triplet states of the substrates, as established by quenching studies involving oxygen, di-tert-butylnitroxide, azulene, and β-carotene.Energy-transfer sensitization of 10, 11, and 23 by benzophenone in benzene also gave rise to the same transients, as formed under direct irradiation.The quantum yieldsof triplet formation (Φ_T) were estimated to be high for 10 and 11 (ca. 1), whereas it is significantly lower for 23 (ca. 0.7).